Heat-assisted magnetic recording (HAMR) involves heating a spot on a disk surface to reduce its coercivity sufficiently so that it can be magnetically recorded. The advantage of this technique is that the coercivity of the media at ambient can be significantly increased, thereby improving thermal stability of the recorded data even for very small bit cells.
One of the difficulties with the technique is finding a method to heat just the small area of media which is to be recorded. Heating with laser light, as is done in magneto-optic recording, is the most promising approach, but the difficulty with this is that at the current storage densities contemplated for HAMR, the spot to be heated is many times smaller than the wavelength of useful semiconductor lasers.
The use of mode index lenses has been proposed to focus light in HAMR applications. However, mode index lenses are susceptible to focusing aberrations due to lens thickness variations, i.e., the refractive index of the lens material changes with lens thickness. Mode index lens fabrication processes often suffer from imprecise thickness control, resulting in unpredictable lens performance and thereby adversely limiting robust and repeatable lens to lens uniformity required for the mass production of HAMR systems. Thus, it would be desirable to focus light with an apparatus not adversely affected by lens thickness variations.
Therefore, there is a need for an improved method and apparatus for focusing energy from a light source suitable for use in HAMR applications.